Online Class taken on 17-04-2020 Topic Unifying characters of Archegoniates Dr. Nilofer Khatoon Assistant Professor in Botany Surendranath College Sources – Internet Archives and Books 17/04/2020 Dr. Nilofer Khatoon 1 Unifying characters of Archegoniates • The archegoniates seem too have originated from a monophyletic group of ancient stock of aquatic green algae. Presence of sexual organs the female archegonium and the male called the antheridium. • The presence of Chloroplasts containing chlorophyll a, b and carotene. • The presence of multicellular gametophytic and sporophytic generation. Heteromorphic alternation of generation. • The morphological reduction of the sexual or the gametophytic phase was evident in the life cycle of archegoniate. Provides protection to their embryo. • Male gametes are flagellated and motile in bryophytes, pteridophytes, (Cycadales, Ginkgoales) while the female gamete (egg) is non-motile. 17/04/2020 Dr. Nilofer Khatoon 2 Unifying characters of Archegoniates • Bryophytes and Pteridophytes depend upon the presence of “fluid water” for fertilization. • In gymnosperms, pollen grains germinate to form a pollen tube (siphonogamy) which is not dependent on external fluid water to reach the archegonial neck. • The transmigration of plants to the land habit led to specialization coupled with varied spore dispersal mechanisms leading to their successful spread on land with genetic variation. • Plants adapted to life on land by internalizing the external atmosphere and exploring the soil in an intensive way. • Spores also became resistant to desiccation through further specialization in seed plants. Differentiated rhizoids and roots to provide strong anchorage and efficient supply of water and mineral nutrients. 17/04/2020 Dr. Nilofer Khatoon 3 Unifying characters of Archegoniates • Increased green surface area to provide more chlorophyll for efficient photosynthesis. • Developed an efficient vascular system to provide water to every part of the plant body. • Evolved mechanism of transpiration to regulate the internal temperature. Developed waxy cuticle to restrict water loss and formed stomata to regulate gaseous exchange. • Differentiated tissues with thickened cell walls (collenchyma) and lignified walls (sclerenchyma) to support the erect habit. Efficient spore dispersal mechanism. • The archegoniates evolved several adaptive strategies to survive on land. 17/04/2020 Dr. Nilofer Khatoon 4 17/04/2020 Dr. Nilofer Khatoon 5 17/04/2020 Dr. Nilofer Khatoon 6 Bryophytes • The alternation of generations is very distinct in the archegoniatae. It appears in the haplodiplontic type of life cycle. • The alternation of generation is slight different in Bryophytes, Pteridophytes and Gymnosperms. • The life cycle of bryophytes shows regular alternation of gametophytic and sporophytic generations. • The haploid phase (n) is the gametophyte or sexual generation. It bears the sexual reproductive organs, which forms gametes, i.e. antherozoids and eggs gametic union a zygote is formed which develops into a sporophyte (2n) diploid phase. 17/04/2020 Dr. Nilofer Khatoon 7 • Sporophyte forms spores, which always germinate to form gametophytes. During the formation of spores, the spore mother cells divide meiotically and haploid spores are formed. The production of the spores is the beginning of the gametophytic or haploid phase. • The spores germinate and form gametophytic or haploid phase. The spores germinate and produce gametophytes, which bear sex organs. • Ultimately, the gametic union takes place and zygote is resulted. It is diploid (2n).This is the beginning of the sporophytic or diploid phase. 17/04/2020 Dr. Nilofer Khatoon 8 • Here the two generations are morphologically different, the type of alternation of generations is called heteromophic. • The gametophytic generation is conspicuous and longer- lived phase of life-cycle in comparison to that of sporophyte generation. • In bryophytes, the gametophyte is quite independent whereas the sporophyte is dependent somehow or other on the gametophyte for its nutrition. • The gametophyte produces sporophyte and sporophyte to the gametophyte and thus there is regular alternation of generations 17/04/2020 Dr. Nilofer Khatoon 9.